1. Field of the Invention
This invention relates to a method for the synthesis of xcex1,xcex2,xcex2-trifluorostyrene and related monomers via the in-situ formation of a trifluorovinyl metal complex.
2. Description of the Related Art
xcex1,xcex2,xcex2-Trifluorostyrenes (xe2x80x9cTFSxe2x80x9d) can be used as monomers in the production of polymers that, in turn, can be used to produce membranes with favorable chemical and mechanical properties. In addition, if the resultant polymers are functionalized with an ion-exchange group, they can be used to form ion-exchange membranes. A polymer membrane comprising TFS and/or substituted TFS monomer units may be suitable for a wide variety of applications and, in particular, such polymer membranes containing ion-exchange functionality have been used in electrochemical applications such as fuel cells as disclosed in U.S. Pat. Nos. 5,422,411, 5,602,185 and 5,834,523.
The synthesis of xcex1,xcex2,xcex2-trifluorostyrene was initially reported in the late 1940""s and early 1950""s. While several methods have since been reported, none of the methods are economically viable in the large scale synthesis of TFS and related monomers. Typical conditions that could render methodologies generally unsuitable for large scale synthesis include low yields, high or low temperatures, high pressures, the use of toxic chemicals and the use of environmentally damaging chemicals such as chlorofluorocarbons (xe2x80x9cCFCsxe2x80x9d).
U.S. Pat. No. 2,612,528 discloses a multi-step synthesis of TFS via a Friedal-Crafts acylation to produce an overall TFS yield of about 30%. In addition to the low yield obtained, the method also requires the use of a toxic fluorinating agent, namely antimony pentafluoride, and the isolation of CFC intermediates.
U.S. Pat. Nos. 2,651,627 and 2,752,400 report a synthesis of TFS from chlorotrifluoroethylene and benzene by pyrolysis at 550-600xc2x0 C. Not only does this method require high temperatures and the use of a CFC as a starting material, but this method only results in low yields of less than 30%. Pyrolysis at 600-800xc2x0 C. was also reported in U.S. Pat. No. 3,489,807 in the synthesis of TFS from xcex2,xcex2-chlorofluoroethylbenzene and 2-chloro-1,1-difluoroethylene, though, relatively low yields were similarly reported. Low yields also result from the synthesis of TFS via the reaction of phenyl lithium with tetrafluoroethylene as disclosed in U.S. Pat. No. 2,874,166. Cryogenic temperatures of xe2x88x9230 to xe2x88x92100xc2x0 C. are disclosed in U.S. Pat. No. 3,449,449 for the reaction of solid phenylsodium with tetrafluoroethylene under high pressure (i.e., 70-1400 kPa) to form TFS.
Relatively high yields at mild temperatures are described in Heinze and Burton (Journal of Organic Chemistry 53:2714-2720, 1998) for the synthesis of TFS. However, this synthesis requires the use of either iodotrifluoroethylene or bromotrifluoroethylene as a starting material. Bromo- and iodotrifluoroethylene are class 2 ozone-depleters that are both relatively expensive and currently commercially available in large volumes from only one source in North America, namely Halocarbon Products Corporation.
Accordingly, there remains a need for improved synthetic methods for making TFS and related monomers, particularly methods that provide for relatively high yields under mild conditions using commercially available and relatively environmentally benign starting materials.
The present method provides for the two-step synthesis of TFS or a derivative thereof from 1,1,2-tetrafluoroethane (xe2x80x9cHFC-134axe2x80x9d).
In the first step, a trifluorovinyl metal complex is formed by effecting a reaction between HFC-134a, an amine, a base and a metal salt. This is shown in the following reaction wherein MX1n is the metal salt: 
The amine may be added to the reaction mixture as a free amine or with the metal salt in a preformed metal salt-amine complex. Alternatively, the amine may be generated in situ. For example, if lithium diisopropylamide is used as the base, diisopropylamine will be generated in situ.
In another embodiment of the first step, a trifluorovinyl metal complex is formed by effecting a reaction between HFC-134a, a base and a metal salt, wherein the reaction temperature is greater than xe2x88x9268xc2x0 C. In a more specific embodiment, the reaction temperature is from about 15xc2x0 C. to about 25xc2x0 C.
The second step involves reacting the trifluorovinyl metal complex, as prepared above, with an aromatic transfer agent (ArX2) in the presence of a metal catalyst and a coordinating ligand to form TFS or a derivative thereof. This is shown in the following reaction: 
The X2 group of the aromatic transfer agent can be any of a variety of suitable transfer agent leaving groups, such as halogen, triflate (i.e., xe2x80x94OSO2CF3), or other suitable groups known to those skilled in the art. In this regard, higher yields have been observed with aromatic iodides. Typically, the aromatic group will be a carbocyclic aromatic group, such as phenyl or naphthyl, although heterocyclic aromatic groups, such as thienyl, may also be used. As discussed in greater detail below, the aromatic transfer agent may be optionally substituted.
To form TFS, the aromatic transfer agent is a phenyl halide (such as phenyl iodide). The metal catalyst may be palladium, nickel or platinum, in either the zero oxidation state or reduced to this oxidation state in situ. Palladium(0) bis(dibenzylidene acetone) is an example of a metal catalyst that is easy to handle and both temperature and air stable. The coordinating ligand can be a mono- or multidentate phosphine, arsine or other ligand known to those skilled in the art. The ligand may be, for example, triphenylphosphine.
The two steps in the synthesis of TFS or derivative thereof can be performed without isolating the trifluorovinyl metal complex intermediate. Further, a mixture of two or more TFS monomers (or derivatives thereof) can be synthesized by adding a second aromatic transfer agent along with the first aromatic transfer agent.
These and other aspects of the invention will be evident upon reference to the following detailed description.